Field
The subject of the application relates to computer networks in general, and more particularly to providing information to networked entities regarding the status of other networked entities.
Description of the Related Art
The Session Initiation Protocol (SIP) is a protocol for creating, modifying, and terminating computer network-based communication sessions, such as for an Internet-based telephone call between two or more participants. Since its introduction, numerous enhancements to SIP have been proposed, including the SIP Instant Messaging and Presence Leveraging Extensions (SIMPLE) framework in which one or more network entities, such as computer users, send subscription requests to a server, known as a “presence” server, to receive presence information regarding another network entity, referred to as a “presentity.” When a change in the presentity's presence information is detected by the presence server, such as when the presentity “publishes” such a change to the presence server, the presence server sends a notification to the subscribers regarding the change.
When a subscription request is made, presence server that handles the subscription forms a subscription dialog that the presence server maintains until the original requestor cancels the subscription or until the subscription expires. The presence server sends notifications to the subscriber regarding any changes to the requested presence information only as long as the subscription dialog is active. Should the subscription dialog become lost or inactive unexpectedly, such as due to failure of the presence server, the subscriber will not know that this has occurred unless some action is taken.